Address comment

Signed-off-by: Lantao Jin <ltjin@amazon.com>

Author: LantaoJin

ppl/src/main/antlr/OpenSearchPPLParser.g4

@@ -14,10 +14,6 @@ root
 
 // statement
 pplStatement
-   : dmlStatement
-   ;
-
-dmlStatement
    : explainStatement
    | queryStatement
    ;
@@ -106,9 +102,7 @@ commandName
    ;
 
 searchCommand
-   : (SEARCH)? fromClause                                           # searchFrom
-   | (SEARCH)? fromClause logicalExpression (logicalExpression)*    # searchFromFilter
-   | SEARCH logicalExpression fromClause                            # searchFilterFrom
+   : (SEARCH)? (logicalExpression)* fromClause (logicalExpression)*     # searchFrom
    ;
 
 describeCommand

ppl/src/main/java/org/opensearch/sql/ppl/parser/AstBuilder.java

@@ -15,9 +15,7 @@
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.FieldsCommandContext;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.HeadCommandContext;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.RenameCommandContext;
-import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.SearchFilterFromContext;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.SearchFromContext;
-import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.SearchFromFilterContext;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.SortCommandContext;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.StatsCommandContext;
 import static org.opensearch.sql.ppl.antlr.parser.OpenSearchPPLParser.TableFunctionContext;
@@ -140,23 +138,16 @@ public UnresolvedPlan visitSubSearch(OpenSearchPPLParser.SubSearchContext ctx) {
   /** Search command. */
   @Override
   public UnresolvedPlan visitSearchFrom(SearchFromContext ctx) {
-    return visitFromClause(ctx.fromClause());
-  }
-
-  @Override
-  public UnresolvedPlan visitSearchFromFilter(SearchFromFilterContext ctx) {
-    return new Filter(
-            ctx.logicalExpression().stream()
-                .map(this::internalVisitExpression)
-                .reduce(And::new)
-                .get())
-        .attach(visit(ctx.fromClause()));
-  }
-
-  @Override
-  public UnresolvedPlan visitSearchFilterFrom(SearchFilterFromContext ctx) {
-    return new Filter(internalVisitExpression(ctx.logicalExpression()))
-        .attach(visit(ctx.fromClause()));
+    if (ctx.logicalExpression().isEmpty()) {
+      return visitFromClause(ctx.fromClause());
+    } else {
+      return new Filter(
+              ctx.logicalExpression().stream()
+                  .map(this::internalVisitExpression)
+                  .reduce(And::new)
+                  .get())
+          .attach(visit(ctx.fromClause()));
+    }
   }
 
   /**

ppl/src/main/java/org/opensearch/sql/ppl/parser/AstStatementBuilder.java

@@ -32,7 +32,7 @@ public class AstStatementBuilder extends OpenSearchPPLParserBaseVisitor<Statemen
   private final StatementBuilderContext context;
 
   @Override
-  public Statement visitDmlStatement(OpenSearchPPLParser.DmlStatementContext ctx) {
+  public Statement visitPplStatement(OpenSearchPPLParser.PplStatementContext ctx) {
     Query query = new Query(addSelectAll(astBuilder.visit(ctx)), context.getFetchSize(), PPL);
     if (ctx.explainStatement() != null) {
       if (ctx.explainStatement().explainMode() == null) {

ppl/src/test/java/org/opensearch/sql/ppl/antlr/PPLSyntaxParserTest.java

@@ -335,17 +335,6 @@ public void testDescribeFieldsCommandShouldPass() {
     assertNotEquals(null, tree);
   }
 
-  @Test
-  public void testDescribeCommandWithSourceShouldFail() {
-    exceptionRule.expect(RuntimeException.class);
-    exceptionRule.expectMessage(
-        StringContainsInOrder.stringContainsInOrder(
-            "[=] is not a valid term at this part of the query: 'describe source=' <-- HERE.",
-            "Expecting tokens:"));
-
-    new PPLSyntaxParser().parse("describe source=t");
-  }
-
   @Test
   public void testInvalidOperatorCombinationShouldFail() {
     exceptionRule.expect(RuntimeException.class);